When a multi layer ceramic capacitor is produced, the following steps are generally adopted. First, a binder resin such as a polyvinyl butyral resin and a plasticizer are added to an organic solvent in which ceramic powders are dispersed and the obtained mixture is homogeneously mixed with a ball mill and the like to prepare a slurry composition for ceramic green sheets. The prepared slurry composition is formed by casting on a strippable support such as a polyethylene terephthalate film, and a solvent and the like are distilled away by heating and the like, followed by stripping from the support to produce a ceramic green sheet.
Next, a plurality of sheets in which a conductive paste, which acts as an internal electrode, is applied on the surface of the ceramic green sheet by screen printing or the like are alternately layered, and a layered product is obtained by thermal compression bonding. Further, a layered product is formed by several steps and cut into a fixed shape. A treatment to remove a binder component and the like contained in this layered product by pyrolysis, what is called degreasing treatment, is carried out, and then by undergoing a step of sintering external electrodes on the end faces of a ceramic fired object obtained by firing, a multi layer ceramic capacitor is produced. Therefore, working properties for excellent preparation operation and strength which can resist several steps are required for the above slurry composition and the above ceramic green sheet, respectively.
In recent years, an increase in capacity and miniaturization of a multi layer ceramic capacitor are desired along with the multi-functionalization and miniaturization of electronic device. In response to this, as ceramic powders used for ceramic green sheets, those with a fine particle diameter of 0.5 μM or less are used, and an attempt to apply them on a strippable support in a thin film state with, for example, 5 μm or less is made.
When ceramic powders with a fine particle diameter are used, however, packing density and surface areas increase. Thus, the amount of binder resin used increases, and along with this, the viscosity of a slurry composition for ceramic green sheets also increases. Therefore, application has become difficult and the poor dispersion of ceramic powders themselves has occurred. On the other hand, in several steps when making a ceramic green sheet, stress such as tension and bending is loaded, and thus a binder resin with a high degree of polymerization is used in order to be able to tolerate such stress.
Patent Literature 1 discloses that a ceramic green sheet obtained from a slurry composition for ceramic green sheets has excellent mechanical strength, wherein the slurry composition for ceramic green sheets contains a polyvinyl acetal resin with a degree of polymerization of more than 2400 and 4500 or less, an amount of vinyl ester unit of 1 to 20 mol % and a degree of acetalization of 55 to 80 mol %, ceramic powders and an organic solvent.
Nowadays, however, further thinning of a ceramic green sheet is desired, and in a case in which a super-thin layer ceramic green sheet is made using the above ceramic slurry composition, when the thickness thereof is 2 μm or less, there has been a problem in that a sheet attack phenomenon easily occurs.
Herein, the sheet attack phenomenon is a phenomenon in which, when a conductive paste, which acts as an internal electrode layer, is printed on the obtained ceramic green sheet, a binder resin contained in the ceramic green sheet is dissolved by an organic solvent in the conductive paste, and defects such as cracks occur on the ceramic green sheet. By the occurrence of this sheet attack phenomenon, the electrical performance and reliability of a multi layer ceramic capacitor deteriorate and the yield ratio significantly decreases.
Patent Literature 2 discloses a polyvinyl acetal resin obtained by acetalization of a polyvinyl alcohol resin with a degree of saponification of 80 mol % or more and a number-average degree of polymerization of 1000 to 4000, wherein the polyvinyl acetal resin composition is characterized in that the degree of acetalization is 60 to 75 mol %, and the ratio of a portion acetalized by acetaldehyde and a portion acetalized by butylaldehyde (the number of moles of hydroxy groups disappeared by acetalization by butylaldehyde/the number of moles of hydroxy groups disappeared by acetalization by acetaldehyde) is 0.1 to 2.
There is, however, a limit on the miniaturization of a multi layer ceramic capacitor, and for an increase in capacity of a chip or miniaturization of a chip with the capacity maintained, not only thinning of a green sheet but also multilayering thereof is desired. Along with such multilayering and miniaturization, there is a problem of hygroscopic properties during storage of green sheets. That is, a binder resin absorbs moisture during storage and thus dimensional change occurs, and thin films are laminated into a multilayer and thus when there is a large amount of water per layer, moisture is quickly evaporated during degreasing and ply separation, called delamination, occurs. Therefore, humidity control during storage of green sheets and the control of degreasing conditions are very important.
Polyvinyl acetal acetalized by acetaldehyde, for example, has a high glass transition temperature and sufficient mechanical strength. Acetaldehyde, however, has low hydrophobicity, and thus a mixed acetalized compound with butylaldehyde exemplified also has high hygroscopic properties and has not satisfied the above problem. In addition, in an acetalized compound by butylaldehyde, low hygroscopic properties have not been sufficiently satisfied.
As described above, Patent Literature 1 and Patent Literature 2 do not disclose a polyvinyl acetal resin having properties which have little dimensional change during storage of green sheets and are less prone to cause ply separation during degreasing.